The salesman said the 150mm would do the job!!!

Hello, I am completely new to photography but I was sold on the OM-D with the 40-150 lens by a great salesman who told me simply that it was just like having a 300mm lens, which is what I was initially after... Long story short, I am not satisfied at all with the zoom on this lens and was wondering how to get more for less! What adapter goes with what lens and also i would like to keep my auto focus as I have read that some adapters stop the communication between camera and lens...
Basically, I want a 300mm or more lens and adapter for under $400!
Any help/suggestions would be great!